Phosphors are used for fluorescent tubes, vacuum fluorescent displays (VFD), field emission displays (FED), plasma display panels (PDP), cathode ray tubes (CRT), white light-emitting diodes (LED), and the like. In these uses, in order to allow the phosphors to emit light, energy for exciting the phosphors needs to be applied to the phosphors. When the phosphors are excited with excitation sources, such as vacuum ultraviolet rays, ultraviolet rays, visible rays, and electron beams, having high energy, the phosphors emit ultraviolet, visible, or infrared light. However, there is a problem in that the brightness of the phosphors is decreased while the phosphors are being exposed to the excitation sources for a long time.
In recent years, various novel materials including nitrides containing three or more elements have been synthesized and have been replacing conventional phosphors such as silicate phosphors, phosphate phosphors, aluminate phosphors, borate phosphors, sulfide phosphors, and oxysulfide phosphors. In particular, the following phosphors have been recently developed: silicon nitride-based phosphors, such as multi-component nitrides and oxynitrides, having excellent properties.
Patent Document 1 discloses a phosphor represented by the formula MxSiyNz:Eu, wherein M is at least one alkaline-earth metal selected from the group consisting of the group Ca, Sr, and Ba and z=2/3x+4/3y. This phosphor is synthesized by a method in which a nitride of the alkaline-earth metal is synthesized by nitridating the alkaline-earth metal and is then mixed with silicon nitride or by a method in which the alkaline-earth metal and silicon imide, which are raw materials, are heated in an N2 or Ar flow. In both methods, the alkaline-earth metal, which is sensitive to air and moisture, needs to be used; hence, the methods are problematic in industrial large-scale synthesis.
Patent Document 2 discloses oxynitride phosphors derived from an oxynitride represented by the formula M16Si15O6N32 and sialons each represented by the formula MSiAl2O3N2, M13Si18Al12O18N36, MSi5Al2ON9, or M3Si5AlON10. In particular, when M is Sr, a phosphor represented by the formula SrSiAl2O3N2:Eu2+ can be obtained in such a manner that SrCO3, AlN, and Si3N4 are mixed at a ratio of 1:2:1 and the mixture is heated in a reducing atmosphere (N2/H2).
In this case, all the phosphors obtained are oxynitrides and any nitrides containing no oxygen cannot be obtained.
A Ca-alpha sialon phosphor, which is an oxynitride phosphor, activated with Eu2+ ions has been proposed (Patent Document 4).
This phosphor is produced by a process below.
A raw material powder is prepared in such a manner that silicon nitride (Si3N4), aluminum nitride (AlN), and europium oxide (Eu2O3) are mixed such that the ratio of Si to Al to Eu is 13:9:1. The raw material powder is fired at 1700° C. for one hour in a 1 atm nitrogen atmosphere by a hot press technique in which the raw material powder is compression-molded at a pressure of 200 atm, whereby an Eu-alpha sialon is produced. Another raw material powder is prepared in such a manner that silicon nitride (Si3N4), aluminum nitride (AlN), and calcium oxide (CaO) are mixed such that the ratio of Si to Al to Ca is 13:9:3. This raw material powder is fired at 1700° C. for one hour in a 1 atm nitrogen atmosphere by a hot press technique in which this raw material powder is compression-molded at a pressure of 200 atm, whereby a Ca-alpha sialon is produced. The Eu-alpha sialon and the Ca-alpha sialon are mixed at a ratio of 50:50. This mixture is fired at 1700° C. for one hour in a 1 atm nitrogen atmosphere by a hot press technique, whereby the target Eu ion-activated Ca-alpha sialon phosphor is produced. It has been reported that the phosphor obtained by this process emits yellow light with a wavelength of 550 nm-600 nm when the phosphor is excited by blue light with a wavelength of 450 nm-500 nm.
Phosphors, excited with ultraviolet rays or blue light, for white LED use or plasma display panel use need to be resistant to degradation during their use.
The raw material powder used to produce the nitride or oxynitride phosphor has low reactivity and therefore is heated in such a manner that the raw material powder is compression-molded at high temperature, that is, the contact area between particles of the raw material powder is increased such that the solid state reaction of the raw material powder is promoted during firing. Hence, the nitride or oxynitride phosphor is obtained in the form of an extremely hard sintered body. The sintered body obtained as described above needs to be finely milled depending on the purpose of the phosphor. However, if the hard sintered body, that is, the phosphor is mechanically milled with, for example, an ordinary jaw crusher or ball mill for a long time with a huge amount of energy, a large number of defects are caused in the host crystal of the phosphor. This causes a problem in that the emission intensity of the phosphor is significantly reduced.
Therefore, the following technique has been attempted: a technique in which the powder is fired without compression-molding the powder. The solid state reaction of the nitride powder does not proceed at low temperature. This is ineffective in producing the target phosphor. Therefore, the phosphor needs to be synthesized at a high temperature of 1800° C. or more. The firing of the powder at such a high temperature causes a problem in that nitrogen is dissociated from the nitride, that is, the nitride is decomposed. In order to prevent this problem, the powder needs to be fired in a nitrogen atmosphere with a pressure of 5 atm or more. This requires not only a large amount of firing energy but also a very expensive high-temperature, high-pressure furnace, causing an increase in the production cost of the phosphor.
In order to synthesize a nitride with low oxygen content, an alkaline-earth metal nitride, for example, calcium nitride (Ca3N2) or strontium nitride (Sr3N2) needs to be used instead of a powder of the alkaline-earth metal. Nitrides of divalent metals are usually unstable in a moisture-containing atmosphere and react with moisture to produce hydroxides. This tendency is particularly remarkable in strontium nitride. Therefore, it has been difficult to produce a phosphor with low oxygen content.
Therefore, a novel production method in which none of the metal nitrides is used as a starting material has been demanded.
Patent Document 3, which relates to methods for producing nitride phosphors from metal materials, has been recently published. Patent Document 3 discloses an example of a method for producing an aluminum nitride-based phosphor and describes that a transition element, a rare-earth element, aluminum, and an alloy thereof can be used as raw materials. However, this document discloses no example in which such an alloy is used as a starting material but discloses that metallic Al is used as an Al source. This method uses a combustion synthesis technique in which a starting material is rapidly heated to a high temperature (3000 K) by igniting the starting material and therefore is significantly different from a method according to the present invention. It is probably difficult to produce a high-performance phosphor by this method. That is, any activating element cannot be uniformly distributed by the technique, in which the starting material is rapidly heated to 3000 K; hence, it is difficult to produce such a high-performance phosphor. This document describes no nitride phosphor containing an alkaline-earth element obtained from the alloy or no nitride phosphor containing silicon.
Known examples of an alloy containing Si and an alkaline-earth metal include Ca7Si, Ca2Si, Ca5Si3, CaSi, Ca2Si2, Ca14Si19, Ca3Si4, SrSi, SrSi2, Sr4Si7, Sr5Si3, and Sr7Si. Known examples of an alloy containing Si, aluminum, and an alkaline-earth metal include Ca(Si1−xAlx)2, Sr(Si1−xAly)2, Ba(Si1−xAlx)2, and Ca1−xSrx(Si1−yAly)2. In particular, A(B0.5Si0.5)2 has been investigated for superconductivity and is disclosed in, for example, Non-patent Documents 1 and 2, wherein A is Ca, Sr, or Ba and B is Al, or Ga. However, there is no example in which any one of these alloys is used as a phosphor precursor. These alloys have been prepared for investigation in a laboratory scale and have not been ever produced in a large industrial scale.
Phosphors, such as Sr(Ca)2Si5N8 and CaAlSiN3, containing Si and an alkaline-earth metal emit yellow to red light when the phosphors are excited with blue or near-ultraviolet light-emitting diodes, as described above. Therefore, the phosphors, which can be used in combination with such blue or near-ultraviolet light-emitting diodes, are industrially useful materials for forming white light-emitting diodes.
However, there has been no method for producing the alloy containing Si and an alkaline-earth metal necessary to produce the phosphors in a large industrial scale. A conventional method for producing the alloy has the following problems: a problem that the alloy contains impurities, a problem that it is difficult to produce the alloy such that the alloy has a designed composition because the alkaline-earth metal has a low boiling point and therefore is readily vaporized, and a problem that the composition of the alloy obtained is nonuniform.
For the production of the phosphor, the presence of impurities in the phosphor impairs luminescent properties of the phosphor even if the amount of the impurities very small. In order to allow the phosphor to have desired luminescent properties, it is essential that, for example, an activating element is uniformly distributed in the phosphor and that the composition of the phosphor is as designed. Therefore, the following method is necessary: a method for producing an alloy for a phosphor precursor in a large industrial scale such that this alloy contains no impurities and the composition of this alloy is as designed and is uniform.
Even if this alloy is obtained, a cast ingot of this alloy is inactive to produce a phosphor. Further investigation is required in order to carry out a desired reaction to convert the alloy into a phospher.
Patent Document 1: PCT Japanese Translation Patent Publication No. 2003-515665
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-206481
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-54182
Non-patent Document 1: M. Imai, Applied Physics Letters, 80 (2002), 1019-1021
Non-patent Document 2: M. Imai, Physics Review B, 68, (2003), 064512
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2002-363554